When a grinding apparatus including a variety of grinding machines is operated to machine a metal such as a ferrous metal, abrasive finishing sludge containing powdery cutting dust and the like is produced. The abrasive finishing sludge is a cumbersome industrial waste which is susceptible to oxidization, because the abrasive finishing sludge contains water, oil and iron and has a microscopic size. There is a demand for processing the sludge into the most possible compact form for recycling. It is therefore a general practice to compress the abrasive finishing sludge by means of a compression machine thereby forming a high-density solid mass. Such a compression machine generally includes: a cylinder body constituting a compaction chamber accommodating a subject material (abrasive finishing sludge); a pressurizing mechanism for pressurizing the subject material toward one end of the cylinder body; and a gate mechanism for opening/closing an aperture at one end of the cylinder body. The compression machine is designed to operate as follows. The abrasive finishing sludge supplied from a hopper disposed upwardly of the cylinder body is carried into the compaction chamber by means of a screw conveyor. The abrasive finishing sludge so delivered is compressed and solidified by means of a hydraulic cylinder constituting the pressurizing mechanism. Subsequently, the above aperture is opened by the gate mechanism, so that the solidified abrasive finishing sludge (briquette) is discharged out of the compaction chamber.
The gate mechanism of the above compression machine includes: a gate member pressed against an end surface at the one end of the cylinder body for closing the aperture of the cylinder body; and driving means for vertically moving the gate member between a first position to close the aperture and a second position to open the aperture. As constantly held in tight contact against the end surface of the cylinder body, the gate member is vertically moved between the first position and the second position.
In the compression of the abrasive finishing sludge in such a compression machine, the pressurizing mechanism applies a pressure in excess of 40 tons to the abrasive finishing sludge, so that the cylinder body constituting the compaction chamber may sometimes be subjected to a pressure in excess of 100 MPa. Therefore, when the gate member is raised, a great frictional force is applied between the solid mass and the gate member in contact with the solid mass due to a residual pressure caused by the spring back of the solid mass. Accordingly, a smooth movement of the gate member is impaired so that the malfunction of the gate member may result. As a solution to this problem, there has been disclosed a compression machine wherein the cylinder body constituting the compaction chamber has a dual structure including an inside and an outside cylinder body, and wherein the inside cylinder body is slightly retreated from the gate member so as to set the solid mass slightly apart from the gate member, whereby the frictional force applied between the solid mass and the gate member is reduced (refer to Japanese Unexamined Patent Publication No. 211599/1998).